Spread spectrum time division duplex (TDD) systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective chip code sequences (codes). In one configuration, TDD systems use repeating frames divided into a number of time slots, such as fifteen time slots. In such systems, a communication is sent in a selected time slot out of the plurality of time slots, and one frame is capable of carrying multiple communications distinguished by both time slot and code. The combination of a single code in a single time slot is referred to as a physical channel. Based on the bandwidth required to support a communication, one or multiple physical channels are assigned to support that communication.
Most TDD systems adaptively control transmission power levels. In a TDD system, many communications may share the same time slot and spectrum. While a wireless transmit and receive unit (WTRU) is receiving a downlink transmission from a base station, all of the other communications using the same time slot and spectrum cause interference to the specific communication. Increasing the transmission power level of one communication degrades the signal quality of all other communications within that time slot and spectrum. Reducing the transmission power level too far results in undesirable signal to noise ratios (SNRs) and bit error rates (BERs) at the receivers. To maintain both the signal quality of communications and low transmission power levels, transmission power control is used.
The purpose of power control is to use the minimum power required to adequately transmit a communication. One measure of power control in TDD for example, may be to use the minimum power to allow each transport channel (TrCH) to operate with a Block Error Rate (BLER) that does not exceed its required level. The standard approach to TDD downlink power control is a combination of inner and outer loop control. In this standard approach, a base station sends a transmission to a particular WTRU. Upon receipt, the WTRU measures the SIR in all time slots and compares this measured value to a target SIR. This target SIR is generated from the BLER signaled from the base station. As a result of a comparison between the measured SIR value and the target SIR, the WTRU transmits a physical layer transmit power control (TPC) command to the base station. The standard approach provides for one TPC command per coded composite transport channel (CCTrCH). The CCTrCH is a physical channel which comprises the combined units of data for transmission over the radio interface to and from the WTRU or base station. This TPC command instructs the base station to adjust the transmission power level of the downlink communication. The base station, which is set at an initial transmission power level, receives the TPC command and adjusts the transmit power level in all time slots associated with the CCTrCH in unison.
An inner loop power control process controls transmit power to maintain the received SIR as close as possible to a target SIR by monitoring the SIR measurements of the data. An outer loop power control process controls the target SIR to maintain the received quality BLER as close as possible to a target quality BLER based on a cyclic redundancy code (CRC) check of the data. The output from the outer loop power control is a new target SIR per CCTrCH used for the inner loop power control.
There are four main error sources in transmission power control: 1) systematic error; 2) random measurement error; 3) CCTrCH processing error; and 4) channel error. The systematic error and the random measurement error are corrected reasonably by the inner loop power control monitoring the SIR measurements. The CCTrCH processing error is corrected by either the outer loop power control or the inner loop power control by using relative SIR measurements among the codes. The channel error is related to unknown time varying channel conditions.
In power control systems, the outer loop power control process sets a target SIR for each CCTrCH based on the required target BLER, assuming a most plausible channel condition. Therefore, the mismatch between the target BLER and the mapped target SIR varies depending on the actual channel condition, and it is especially large at very low BLER. Since the outer loop power control depends on the CRC check, it often takes a long time to converge to the required target SIR for the low BLER.
Accordingly, there is a need for outer loop power control which determines the actual channel conditions so that a proper value for the target SIR is used.